This invention generally relates to a fuel delivery system for an aircraft turbine engine, and specifically to a fuel delivery system including a fuel de-aerator for removing dissolved gasses within fuel prior to entering a main fuel pump.
A fuel delivery system for a gas turbine engine typically includes a tank boost pump that pumps fuel from a fuel tank to a two-stage main fuel pump. The main fuel pump typically includes a centrifugal stage and a positive displacement stage. In most applications, the main fuel pump and tank boost pump are driven by an engine drive shaft. The main fuel pump centrifugal stage supplies necessary pressure to the inlet of a positive displacement gear stage. Pressure at the inlet of the gear stage is required to fill the cavities of the gear pump with fuel as the gears rotate. Gears rotate at a constant rotational speed to provide a constant flow of fuel to a fuel-metering device. The fuel-metering device receives flow from the main fuel pump at a constant rate independent of system backpressure. The fuel-metering device controls the flow rate of the fuel that is delivered to the engine. At lower flow rates the excess fuel flow at the fuel-metering device is bypassed back to the gear pump inlet.
The fuel system for the turbine engine is limited by the range of fuel flow rates that are capable of supplying a net positive suction pressure required in the centrifugal stage of the main fuel pump. The net positive suction pressure defines the minimum total pressure required at the pump inlet for the pump to operate without cavitating. Cavitation results when the pressure along the pump vane drops low enough for dissolved gases to form vapor bubbles. The pressure increases as the fluid flows along the pump vane causing the vapor bubbles to collapse. Cavitation in the pump is not desirable because the collapsing vapor bubbles can cause excessive noise and vibration at the main fuel pump. De-aerating the fuel delays the onset of cavitation, thereby increasing the range of fuel flows that can be delivered by a fuel system to a gas turbine engine without altering the design of the fuel pump.
Fuel stored in the fuel storage tank is in direct contact with air and accumulates a quantity of dissolved gases that are mostly oxygen and nitrogen. Static pressure of the fuel is reduced as the fuel flows through the system causing dissolved gases to be released from the fuel forming vapors that flow along with the liquid fuel. Disadvantageously, dissolved gases within the liquid fuel increase the net positive suction pressure required at the main pump inlet.
Accordingly it is desirable to design a fuel delivery system that includes a device for removing gases from fuel prior to entering the main pump inlet to reduce the required net positive suction pressure thereby suppressing cavitation and increase the range of operable fuel flow rates.